System and method for calibrating a high resolution data acquisition system with a low resolution digital to analog converter

Abstract

A high resolution data acquisition (DAQ) system is initially calibrated with a reference source having a resolution higher than the DAQ system. Measurements over the operating range of the DAQ system are taken, and characteristic calibration coefficients are determined from the measurements. Software corrections based upon the calibration coefficients are made to DAQ system measurements. A digital to analog converter (DAC) on-board the DAQ system having a lower resolution than the DAQ system is calibrated by generating a look-up table of input digital codewords and output electrical signals measured by the calibrated DAQ system. This look-up table is used to field calibrate the DAQ system using only the DAC, rather than the high resolution reference source.

What is claimed is: 
     
       1. A data acquisition (DAQ) system having a first resolution and comprising a digital to analog converter (DAC), wherein the DAC has a lower resolution than the first resolution and is configured to be used as a source for calibrating the DAQ system after initially calibrating the DAQ system and the DAC with a reference source having a higher resolution than the first resolution. 
     
     
       2. The data acquisition system of  claim 1  wherein initially calibrating the DAQ system comprises:
 generating a first set of data points obtained by sending a first set of electrical signals to the DAQ system from the reference source and measuring a first set of output electrical signals from the DAQ system; 
 generating a first set of calibration coefficients for the DAQ system from the first set of data points; 
 applying a first software correction to measurements of the DAQ system based upon the first set of calibration coefficients; and 
 further wherein initially calibrating the DAC comprises:
 generating a second set of data points obtained by sending digital codewords to the DAC to generate a second set of electrical signals from the DAC to the DAQ system and measuring a second set of output electrical signals from the DAQ system, wherein the first software correction has been applied; and 
 storing the second set of data points for using the DAC as the source for calibrating the DAQ system. 
 
 
     
     
       3. The data acquisition system of  claim 2  wherein generating a first set of calibration coefficients comprises fitting a curve to the first set of data points and determining a slope and an offset of the curve. 
     
     
       4. The data acquisition system of  claim 3  wherein fitting a curve to the first set of data points comprises using a least squares fitting algorithm. 
     
     
       5. The data acquisition system of  claim 2  wherein using the DAC as the source for calibrating the DAQ system comprises:
 generating each digital codeword in the second set of data points and feeding it to the DAC, wherein an output of the DAC couples to an input of the DAQ system; 
 recording each corresponding measured output from the DAQ system; 
 determining a second set of calibration coefficients from the digital codewords and corresponding measured outputs; and 
 applying a second software correction to the first software correction to measurements of the DAQ system based upon the second set of calibration coefficients. 
 
     
     
       6. The data acquisition system of  claim 2  wherein the DAQ system has multiple channels, and further wherein each channel has a first set of data points, a first software correction, a first set of calibration coefficients, and a second set of data points. 
     
     
       7. The data acquisition system of  claim 6  wherein one or more of the multiple channels provide a gain. 
     
     
       8. The data acquisition system of  claim 1  wherein the DAC has a low temperature coefficient. 
     
     
       9. A data acquisition (DAQ) system having a first resolution that is calibrated by a digital to analog converter having a lower resolution than the first resolution, comprising:
 a signal chain configured to accept an input electrical signal and provide signal conditioning; 
 a digital to analog converter (DAC) coupled to an input of the signal chain, wherein the DAC is configured to accept a digital codeword input and output a corresponding analog electrical signal to the input of the signal chain, and further wherein the DAC has a second resolution that is lower than the first resolution; 
 an analog to digital converter (ADC) coupled to an output of the signal chain; 
 a memory configured to store a first software correction to measurements of the DAQ system based upon calibration of the DAQ system by a reference source having a resolution higher than the first resolution and a set of data obtained by sending a set of digital codewords to the DAC to generate a set of electrical signals from the DAC to the signal chain and measuring a set of output electrical signals from the ADC; and 
 a processor configured to control the DAC based at least upon the set of data to calibrate the DAQ system, wherein calibrating the system comprises:
 generating each digital codeword in the set of data and feeding it to the DAC, 
 recording each corresponding measured output from the DAQ system, 
 determining calibration coefficients from the digital codewords and measured outputs, and 
 applying a second software correction based upon the set of calibration coefficients to calibrate the DAQ system, 
 
 wherein the DAQ system is initially calibrated with a reference source having a third resolution that is higher than the first resolution to generate the first software correction. 
 
     
     
       10. The system of  claim 9  wherein a least squares fitting algorithm is used to determine the calibration coefficients from the digital codewords and measured outputs. 
     
     
       11. The system of  claim 9  wherein the DAC has a low temperature coefficient. 
     
     
       12. The system of  claim 11  wherein the signal chain comprises multiple channels, and a set of calibration coefficients is stored in memory for each channel. 
     
     
       13. The system of  claim 12  wherein one or more channels in the signal chain has a gain. 
     
     
       14. The system of  claim 9  wherein calibration with the reference source having a third resolution higher than the first resolution comprises:
 generating a first set of data points obtained by sending a first set of electrical signals to the signal chain from the reference source and measuring a first set of output electrical signals from the ADC; 
 generating a first set of calibration coefficients for the DAQ system from the first set of data points; and 
 applying the first software correction to measurements of the DAQ system based upon the first set of calibration coefficients. 
 
     
     
       15. A method of calibrating a data acquisition (DAQ) system using a reference source having a lower resolution than the DAQ system, comprising:
 accessing a first set of data comprising digital codewords and corresponding measured DAQ system outputs, wherein the first set of data was obtained by:
 pre-calibrating the DAQ system with a precision reference source having a higher resolution than the DAQ system, 
 sending each digital codeword to a digital to analog converter (DAC) to generate a first set of electrical signals from the DAC to an input of the DAQ system, wherein the DAC has a lower resolution than the DAQ system, and 
 measuring a first set of output electrical signals from the DAQ system; 
 
 sending each digital codeword to the DAC to generate a second set of electrical signals from the DAC to the input of the DAQ system; 
 measuring each second output electrical signal from the DAQ system; 
 recording as a second set of data each measured output electrical signal from the first set of data and each measured second output electrical signal; 
 fitting the second set of data to a curve; 
 determining calibration coefficients from the curve; and 
 applying software corrections to the DAQ system based at least upon the determined calibration coefficients. 
 
     
     
       16. The method of  claim 15  wherein the DAQ system comprises multiple channels, and each channel is calibrated independently. 
     
     
       17. The method of  claim 16  wherein one or more of the channels provide a gain. 
     
     
       18. The method of  claim 15  wherein pre-calibrating the DAQ system comprises:
 generating a reference set of data points obtained by sending a reference set of electrical signals to the DAQ system from the precision reference source and measuring a reference set of output electrical signals from the DAQ system; 
 generating a reference set of calibration coefficients for the DAQ system from the reference set of data points; and 
 applying a reference software correction to measurements of the DAQ system based upon the reference set of calibration coefficients. 
 
     
     
       19. The method of  claim 15  wherein the DAC has a low temperature coefficient. 
     
     
       20. The method of  claim 15  wherein a least squares fitting algorithm is used for fitting the data to the curve.